The present invention relates to the treatment of glasses known in the art as 96% silica glasses, and particularly to the treatment of such glasses to modify the physical properties thereof by removing hydroxyl ions therefrom.
Glasses of the kind referred to as 96% silica glasses, and methods for manufacturing such glasses, are described in U.S. Pat. No. 2,106,744 to Hood et al. In accordance with such methods, alkali borosilicate glasses of specified composition are subjected to a phase separation heat treatment to separate the glass into a silica-rich phase and a phase rich in alkali and boron. This latter phase is then removed by leaching to provide a microporous glass body comprising a very high percentage of silica (typically at least about 95% by weight) and minor amounts of residual alkali and boron.
Glass produced by this process is referred to as 96% silica glass without regard to the exact silica content thereof, and may be used either in the porous state or after consolidation to the non-porous state by heating. In porous form, the glass may be used to support catalysts and enzymes, and as a chromatographic medium. Consolidated 96% silica glass is used to provide refractory glass articles such as crucibles and envelopes for arc lamps.
A number of methods are known for treating 96% silica glass in the porous state to modify glass properties for certain applications. U.S. Pats. Nos. 3,113,008 and 3,113,855, for example, describe methods for treating porous 96% silica glass to increase the annealing point thereof, while U.S. Pat. No. 3,804,647 describes stabilized porous glass catalyst support materials. U.S. Pat. No. 3,782,915 describes the use of ammonia and steam treatments to deactivate Lewis acid sites attributable to boron on porous glass to be used for chromatography, and it is known that boron can be expelled from porous glass by heating in a steam atmosphere.
It is also known that glasses generally can include hydroxyl species such as, for example, silanol groups (--Si--OH). These can result from the presence of hydroxides or water in batch materials, or from moisture otherwise introduced during glass processing. Such species not only reduce the annealing point of the glass but also give rise to infrared absorption bands therein.
Prior art methods of "dewatering" porous glass to remove hydroxyl species therefrom include firing treatments in a dry environment (e.g., dry air or vacuum) or treatment with chemical dehydroxylating agents such as chlorides or fluorides. U.S. Pat. No. 2,982,053 discloses fluoride treatments for dewatering porous glass, while U.S. Pat. No. 3,459,522 describes chloride treatments for this purpose.
Vacuum firing is inconvenient, while dry air treatments are not as effective in reducing the concentration of hydroxyl species as might be desired. Chloride and fluoride dehydroxylating agents introduce halogen ions into the glass which, unless properly controlled, act as fluxes to reduce the annealing point thereof.
It is a principal object of the present invention to provide a convenient process for dewatering porous glass through the removal of hydroxyl species therefrom which is quite effective in increasing the annealing point and reducing the infrared absorption of the glass.
It is a further object of the present invention to provide a dewatering process which does not require a vacuum firing step or the use of fluxing chemical drying constituents to achieve hydroxyl group removal.
Other objects and advantages of the invention will become apparent from the following description thereof.